A memristor is a circuit element that can remember its previous state[1, 2]. Its main fingerprint is a pinched-hysteresis loop when subjected to bipolar periodic stimuli. This fingerprint has been observed in nanoionics [3-5], metal-insulator transitions [6, 7], and ferroelectric tunneling [8, 9]. The discovery of nanoionic memristive signals has prompted much research because of possible applications of memristors in terabit memories [10, 11], logic operators [12], and neuristors [13, 14].
A physical mechanism which is responsible for memristive behavior of conventional nanoionic memristors has been reported [1, 3-5, 10, 11]. The mechanism is believed to involve coupled electron-ion dynamics involving changes in the electronic barrier at an interface under an electric field. These changes are believed to result from oxygen deficient channels in the material. Single-phase binary or ternary metal oxides in their virgin states do not contain these channels, but application of a suitable voltage to the virgin sample has been found to initiate memristive behavior [15, 16]. The application of a voltage or current to the virgin sample suitable for initiating memristive behavior is known in the art as ‘electro forming’. Nanoionic circuit elements that can operate at room temperature have not yet been prepared by any process other than electroforming.
However, there are problems associated with the use of electroforming to provide memristive behavior because electroforming is a destructive process with a random and uncontrollable nature [11, 15, 17]. Samples may be damaged or destroyed by the high voltage or current [15], and memristors prepared by electroforming may also suffer from problems of non-uniformity and non-reproducibility [11, 17].
Other approaches besides electroforming have been explored for the creation of these oxygen deficient channels in metal oxide samples. One alternative approach for forming oxygen deficient channels in single phase oxide materials is a partial substitution approach [18, 19] that has been used mainly for preparing oxide electrolytes for solid oxide fuel cells and oxygen sensors that operate at temperatures above 650° C. The other approach involves the preparation of lateral multilayered structures [20]. Although this second approach provides large concentrations of oxygen vacancies distributed throughout lateral interfaces [21, 22], it is not readily adaptable for preparing circuit elements because current flowing in lateral directions results in both the poor integration density and the processing difficulty in device fabrication.